Passage
The influenza A virus is an enveloped RNA virus with potential for zoonosis (transmission from animals to humans) . It obtains its plasma membrane from its host cell, but the composition of the viral envelope and vertebrate plasma membrane are distinct. The envelope of H6-G225D consists mostly of phosphatidylethanolamine as well as some negatively charged glycerophospholipids. The outer membrane contains hemagglutinin (HA) proteins that bind to the carbohydrate known as sialic acid (SA) on host cell receptor proteins, facilitating viral entry. It also contains neuraminidase enzymes that break the bond between SA and galactose in the host membrane glycans, allowing new virions to be released.Minute changes in the genes that code for HA proteins can alter the virulence, host, and attachment mechanism. In general, influenza A infects avian species by attaching to tracheal tissue, which displays SA groups bound to receptors through an α-2,3-glycosidic linkage to galactose (Figure 1) . Avian viruses generally cannot attack human cells, in which SA is linked to galactose through an α-2,6-glycosidic linkage instead of an α-2,3 linkage.
Figure 1 Structure of the influenza A receptor in avian cell membranesIn a recent study, however, a G225D substitution in HA, caused by a GGC to GAU codon mutation, enabled avian virions to infect human cells. The stability of G225D HA proteins was compared to that of H6-P186L, which is created by a single CCC to CUC mutation and exclusively infects avian cells, and H1N1, which targets only humans. The viral RNA of H6-P186L, H6-G225D, and H1N1 was cloned using cDNA and expressed in embryonic kidney cells. The melting curve of each purified HA protein was determined using differential scanning calorimetry, with peaks representing melting temperatures (Figure 2) .
Figure 2 Hemagglutinin melting curves for H6N1 and H1N1 influenza A strains
Adapted from De vries RP, Tzarum N, Peng W, et al. A single mutation in Taiwanese H6N1 influenza hemagglutinin switches binding to human-type receptors. EMBO Mol Med. 2017;9(9) :1314-1325.
-The absence of which of the following lipids is most likely to affect the assembly of viral envelopes?

Question

Passage
The influenza A virus is an enveloped RNA virus with potential for zoonosis (transmission from animals to humans) . It obtains its plasma membrane from its host cell, but the composition of the viral envelope and vertebrate plasma membrane are distinct. The envelope of H6-G225D consists mostly of phosphatidylethanolamine as well as some negatively charged glycerophospholipids. The outer membrane contains hemagglutinin (HA) proteins that bind to the carbohydrate known as sialic acid (SA) on host cell receptor proteins, facilitating viral entry. It also contains neuraminidase enzymes that break the bond between SA and galactose in the host membrane glycans, allowing new virions to be released.Minute changes in the genes that code for HA proteins can alter the virulence, host, and attachment mechanism. In general, influenza A infects avian species by attaching to tracheal tissue, which displays SA groups bound to receptors through an α-2,3-glycosidic linkage to galactose (Figure 1) . Avian viruses generally cannot attack human cells, in which SA is linked to galactose through an α-2,6-glycosidic linkage instead of an α-2,3 linkage.

Figure 1 Structure of the influenza A receptor in avian cell membranesIn a recent study, however, a G225D substitution in HA, caused by a GGC to GAU codon mutation, enabled avian virions to infect human cells. The stability of G225D HA proteins was compared to that of H6-P186L, which is created by a single CCC to CUC mutation and exclusively infects avian cells, and H1N1, which targets only humans. The viral RNA of H6-P186L, H6-G225D, and H1N1 was cloned using cDNA and expressed in embryonic kidney cells. The melting curve of each purified HA protein was determined using differential scanning calorimetry, with peaks representing melting temperatures (Figure 2) .

Figure 2 Hemagglutinin melting curves for H6N1 and H1N1 influenza A strains
Adapted from De vries RP, Tzarum N, Peng W, et al. A single mutation in Taiwanese H6N1 influenza hemagglutinin switches binding to human-type receptors. EMBO Mol Med. 2017;9(9) :1314-1325.
-The absence of which of the following lipids is most likely to affect the assembly of viral envelopes?

Quizplus · Accepted Answer

11ecba2d_0eb7_e548_a3bf_43ba0c280a87_MD0008_00 The plasma membrane is composed mainly of proteins, phospholipids, and cholesterol arranged in a bilayer with hydrophilic surfaces enclosing a hydrophobic interior. Phospholipids have a wide range of structures but generally consist of one or more fatty acid tails linked to a polar head group that contains a phosphate. The charges of phospholipids depend on the head groups attached to the phosphate. Head groups can be either neutral or negatively charged. Influenza A obtains its viral envelope from the plasma membrane of its host cell. However, the passage states that the composition of the envelope differs significantly from the plasma membrane. Unlike human membranes, the viral envelope contains mostly phosphatidylethanolamine and negatively charged phospholipids. Therefore, phospholipids with ethanolamine and negatively charged head groups such as serine, inositol, and diphosphatidylglycerol are needed for envelope formation. Of the choices presented, phosphatidylserine is the only negatively charged phospholipid. Therefore, its absence is most likely to affect the assembly of the viral envelope. (Choice A) Palmitic acid is the only free fatty acid synthesized by cells. Although it can be incorporated into phospholipids, the passage does not indicate that it is required. (Choice B) Triglycerides are storage lipids found in adipocytes (fat cells) and consist of three fatty acid tails bound to a glycerol backbone by ester linkages. They are not found in cell membranes or viral envelopes. (Choice C) Sphingomyelin is a phospholipid with a sphingosine backbone. The phospholipids required for the viral envelope (phosphatidylethanolamine and negatively charged glycerophospholipids) do not contain sphingosine, but instead contain a glycerol backbone. Sphingomyelins are not part of the viral envelope. Educational objective: Phospholipids are composed of a hydrophilic polar head that contains a phosphate group and a hydrophobic tail with one or more fatty acids. They may contain a glycerol or a sphingosine backbone, and their properties depend on the head groups attached to the phosphate.